The present invention relates to an endoscope system, a control method, an imaging device, and the like.
An imaging device such as an endoscope may be required to generate a deep-focus image inside a body cavity in order to facilitate a doctor's diagnosis. However, the object captured using an endoscope does not necessarily have a planar shape, but may have a tubular shape (e.g., large intestine or bronchial tube). Therefore, a deep-focus image has been implemented by increasing the depth of field of an endoscope by utilizing an optical system having a relatively large aperture value (hereinafter may be referred to as “F-number”).
In recent years, a megapixel image sensor has been used for an endoscope system. However, the size of the image sensor provided on the end of the endoscope system is limited, and it is necessary to reduce the pixel pitch (vertical and horizontal dimensions of one pixel) in order to increase the number of pixels. The depth of field of an optical system is determined by the size of the permissible circle of confusion. Since an image sensor having a large number of pixels has a small pixel pitch and a small permissible circle of confusion, the depth of field of the imaging device decreases. In this case, the depth of field may be maintained by increasing the aperture value of the optical system. According to this method, however, the optical system darkens, and noise increases, so that the image quality deteriorates. Moreover, the effects of diffraction increase as the aperture value increases, so that the imaging performance deteriorates. Accordingly, a high-resolution image cannot be obtained even if the number of pixels of the image sensor is increased.
JP-A-6-342122 discloses a technique that increases the depth of field and the intensity of light of an endoscope system that includes a variable aperture and has the above problem. According to the technique disclosed in JP-A-6-342122, the range of the depth of field can be observed by appropriately synchronizing the focus function and the aperture operation.
JP-A-8-181909 discloses a technique that suppresses a change in resolution based on the aperture value. In JP-A-8-181909, a constant resolution is obtained independently of the aperture value by decreasing the degree of contour enhancement when decreasing the aperture value of the optical system, and increasing the degree of contour enhancement when increasing the aperture value of the optical system.